The universe is indeed depleting its supply of hydrogen, but given that the baryonic (i.e., 'normal' matter, not dark matter or dark energy) mass of the universe was ~77% hydrogen a few minutes after the Big Bang and now, 13.7 billion years later, it's only down to ~74%, the universe itself won't run out of hydrogen.

However: Galaxies themselves do run out of fresh hydrogen gas. A large portion of the hydrogen in the universe (maybe most of it, I'm not sure off the top of my head) is in the diffuse Intergalactic Medium (IGM), floating in the deep dark spaces between galaxies, spaces which are only getting larger and emptier as the universe expands. Most of the IGM will never fall into galaxies.

Could this outpace entropy/heat death? Or is there something that rules this out (e.g. sheer quantity of hydrogen, some opposing fissile force)?

I'm really not sure what you're asking here.

edit: No_Pepper has reminded me that if the proton has a half-life, as some theories predict it should, then eventually all the hydrogen in the universe will decay. Current experiments have failed to detect proton decay, meaning that if it does have a half-life, then it's over 1034 years, which is a truly tremendous amount of time.

All elements in the universe naturally progress towards iron. The lighter (than iron) ones fuse to release energy (become more stable) whilst the heavier ones split (fission) to release energy. This is why both fusion and fission can be used to generate power.

I assume this means that eventually the universe should tend to an all iron equilibrium, after which I guess baryon decay would dominate.

After all, it is differences in quantities which allow you to do anything interesting. Feynman: you can only use towels to dry things that are wetter than they are, eventually everything has the same wet-potential and then your towels are useless. (paraphrased )

I assume this means that eventually the universe should tend to an all iron equilibrium, after which I guess baryon decay would dominate.

This would be true if all the matter were endlessly recycled into stars. Most baryonic matter will remain hydrogen and helium, because much of it won't make it into stars, and even when it does make it into stars, stars only convert something like 10% of their hydrogen mass to helium during the main sequence, and then later in life they convert some of the helium to heavier elements (the rate depends on the star's mass).

I know this is an old paper, but Freeman Dyson postulated that, in a universe without proton decay (which has yet to be observed) all matter will decay to iron-56 on the order of 101500 years via quantum tunneling. Has this been refuted since this paper was published?

You know, I've never heard of that paper before, but I think his calculations would probably be changed somewhat by our relatively new knowledge that the universe's expansion is accelerating, so matter is getting farther and farther apart (in fairly exponential fashion) and it will become harder and harder for that quantum tunneling to occur.

This is interesting. What I want to know is who wins in the acceleration vs quantum tunnelling battle? Yes, the likelihood will decrease...but given an infinite amount of time the series (integral?) might diverge. It's been a while since I've actually done QM.

So I have not read the paper or done any calculations but I believe the current (Lambda-CDM) model for the universe wouldn't allow for communication of information between the majority of particles long before that 101500 yr limit. In addition to this, that timescale is well beyond the expected decay time for protons so is highly speculative.

The universe isn't expanding into anything. The universe itself is simply getting larger. Any given parcel of space (this is applicable only on large scales, as in millions+ lighyears) is continuously getting larger.

Additional clarification: gravity is stronger than the rate of expansion, as are the nuclear forces and electromagnetic force. Which is why the space between stars and planets or inside of atoms isn't increasing, but the space between galaxies and clusters of galaxies is.

The two outstanding theories on wikipedia are that the fate of the universe is either a motionless void ice death, or an increasingly fiery inferno, both of which are irreversible scenarios with all life long dead.

There's a chance that as the Sun expands and loses its outer envelope, the decreasing mass will make the Earth's orbit spiral out enough that it doesn't quite get swallowed, but it still roasts and loses its atmosphere.

"As the Sun expands, it will swallow the planets Mercury and, most likely, Venus.[106] Earth's fate is less clear; although the Sun will envelop Earth's current orbit, the star's loss of mass (and thus weaker gravity) will cause the planets' orbits to move farther out.[97] If it were only for this, Venus and Earth would probably escape incineration,[102] but a 2008 study suggests that Earth will likely be swallowed up as a result of tidal interactions with the Sun's weakly bound outer envelope.[97]"

Yes I believe during Wonders of the Universe with Brian Cox, he was explaining the heat death and that it won't happen for 10 then he said trillion about 6 times. There aren't even close to that many atoms in the universe.

According to Wikipedia, the heat death of the universe is at least 10100 years away, so 10 with about "8 trillions". You cant really compare years to atoms though, and there are estimated to be ~1080 atoms in the observable universe (again according to Wikipedia, lazy research I know), so yes I guess you could say there are way more years left until heat death than atoms in the universe...

If your heart needs warming, there is always the theory of Multiverse. This is just one of almost infinite Universesin a boiling pot, perhaps there is one Universe that has been able to overcome heat death.

The "binding energy" is basically how strongly the nucleus is held together by the strong force. The bottom axis is just the elements from hydrogen on the left all the way to uranium and very heavy things on the right. You can see iron (Fe 56) chilling at the top of this curve. Iron-56 has the highest binding energy per nucleon.

Conversely, you can flip this graph over and plot it as potential (so that it looks like a "U" shape, with iron at the bottom). This potential works exactly like the gravitational potential of a hole in the ground. Over time, things tend to end up at the bottom of the hole because it's more stable.

The change in binding energy going from one nucleus to another is emitted as radiation etc, so we can see that the lighter elements have to INCREASE in mass (ie. fuse together) to achieve this, whilst the heavier elements must split apart to lose potential energy. Both these processes result in it becoming more "iron-like". The only extrapolation really necessary is to say that in an infinite universe this process would continue until everything is iron...maybe (read other comments, this is the basics though).

It's not that there's any special "equilibrium" property of iron, it's that the iron-56 nucleus has a lower binding energy than any other nucleus, so quantum tunneling will eventually tend all matter to iron. This is predicted to take about 101500 years.

That sounds a lot like an equilibrium - i.e. if it's manganese it could be fused, if it's cobalt it can be fissioned with net energy gain (ignoring, for the moment, other isotopes and Ni-67). I hadn't thought about it before and it really seems like quite a big deal to be the center below which anything else or anything above has spare entropy that can be converted to energy.

Not saying that means you need to have an answer, in fact I'm just happy someone mentioned that. But that seems like an awfully special point.

Greatest magnitude. Typically binding energies are given as a negative energy with respect to setting the potential energy equal to zero at an infinite separation distance, so saying "lowest" is accurate in that sense (e.g. binding an electron to a free proton to create neutral hydrogen releases 13.6 eV of energy, so w.r.t. the unbound state, ΔE = -13.6 eV).

Silicon-28 is fused with one helium nucleus after another, until reaching Nickel-56, which decays to Cobalt-56 and then Iron-56. It stops here because, although Ni-62 and Iron-58 are slightly more stable, it would first require intermediate steps which involve much less stable isotopes. The fusion process doesn't have any foresight saying "it'll all be worth it in the end," so there's no incentive to progress through those intermediates.

To a certain extent, the presence of metals (in astronomy, the term 'metals' refers to anything heavier than helium, not just elements which are chemically metals) prevents the formation of extremely high-mass stars (>120 solar masses) because metals can radiate heat away and speed the collapse of a cloud before it gets truly humongous, but I'm not sure how they affect the Initial Mass Function of stars in general.

Not all the fuel will be spent, because most of it won't make it into stars at all. The universe is getting emptier and colder and darker all the time, although galaxies and galaxy clusters will remain bound together.

But this 1034 calculation is the lower limit based upon bound matter. For example, a free neutron has a half-life of ~15 minutes. Once it is bound in an atom with protons and other neutrons, it is incredibly stable. Proton decay is completely theoretical, but we need it to happen to make all of our math work from just after the Big Bang to the present day.

If, however, the proton does decay, then the universe will eventually be devoid of matter, meaning that the net entropy of the universe is 0.

Unlike a neutron, which decays into a proton and an electron, a proton would not be able to decay into lighter baryonic matter. Right now, we're mostly looking for them to decay into a neutrino, a pion, and a positron. When positrons collide with other protons, they annihilate into gamma rays, which further reduces the number of protons in the universe. It would take something like 10101010 years, but we expect the universe to eventually be devoid of anything except photons and neutrinos. (Again, this is all if the proton has a half-life.)

Nice username by the way! You probably haven't read many of my papers, since I don't have very many yet (still in grad school doncha know!) but radio astronomy is a small field...

As for gas getting into galaxies, my personal opinion is that it's almost entirely from accretion of gas-rich dwarfs. As best I can tell the IGM is generally too hot and diffuse to passively accrete with any efficiency. Of course, HI clouds come in all sizes, from gas-rich dwarfs to tiny objects like the high-velocity clouds that have been found around the Milky Way (Bart Wakker is the guy for this) and which are being found around other nearby spirals as well. Then there are a few weirdos like good ol' NGC 891 which have huge diffuse extraplanar emission whose origin is rather puzzling.

Gas rich dwarfs don't have enough fuel to supply gas to galaxies; if you do an accounting of the star formation rate and the number counts of such galaxies it doesn't add up. There's some nice work counting larger dwarfs, which have to supply most of the gas if lambdaCDM is to be believed, by Tollerud and the folks at UCI. Most people believe there is some mix of cold flows and hot accretion in the CGM that drives accretion, of which HVCs (my thesis topic) are a part.

Hawking Radiation is a theorized method by which black holes can evaporate over long periods of time. It takes an extremely long time for a black hole to evaporate, and the bigger the black hole, the longer it takes. All the stars in the universe will be long dead before Hawking radiation does much to change the mass of black holes in the universe. If you want to read more, a quick search of this sub should turn up some decent discussions of it.

As far as I'm aware there has been no confirmed detection of Hawking radiation. Wikipedia seems to agree, for what it's worth. Black holes aren't my field, but I suspect I would have heard if Hawking radiation had been detected.

Physicist here. Hawking radiation has definitely not been observed. You're thinking of a condensed matter experiment where the the variables are tuned right so, if you squint your eyes kinda, the system looks mathematically reminiscent of black hole in GR. But the dynamics of these systems are all ultimately based on low-energy QM and so can't tell us anything about quantum gravity effects like Hawking radiation.

Incidentally, Hawking would have one of the most immediate and assured Nobel prizes in history if it ever was observed.

This is not a direct answer to your question but it touches it and is an interesting read. May give a sense of scale for certain cosmic events and such… or it might just boggle your mind by the sheer amount of time passing. Timeline of the far future

Iron has the highest nuclear binding energy. The nuclear force that holds its nucleus together has the best balance with the repulsive electrostatic force between its protons. Smaller atoms want to fuse to become more like iron, larger atoms want to undergo fission to become small like iron. (I describe it as though they make a conscious effort. Obviously that is not the case, but it's a simple way to think about it).

Think of it in terms of equilibrium. In large stars, the energy of fusion itself is what prevents the star from collapsing under it's own immense weight. When the core runs out of fusible elements, it can no longer push back against the force of gravity, and collapses in a Supernova. For that split second, an immense amount of energy is being focused on the core, enough to fuse over and over into all of the elements heavier than Iron. Then it rebounds, and scatters the results of this process across space.

Regular stars only have the energy to make elements up to iron and nickel. Supernovas have the energy to fuse these further. After a certain size, the atoms become too unstable and begin to decay back into smsller ones.

Read Death From The Skies by Phil Plaitt. He covers what happens inside stars, and the mechanics of supernovae and the creation of heavier elements that iron. It's absolutely fascinating, fun to read and very easy on the brain.

Look at that graph. It's a graph of nuclear binding energy per nucleon. Nuclear binding energy is the energy required to completely disassemble an atom. When you take a bunch of stuff with low binding energy and convert it into a bunch of other stuff with high binding energy, you have left over energy. That energy is released as heat. When you convert a bunch of hydrogen (stuff with low binding energy) into iron (stuff with high binding energy) you release heat. When you convert a bunch of uranium (stuff with low binding energy) into iron (stuff with high binding energy) you release heat. From a very high level thermodynamic standpoint, fusion and fission are the same thing: you're converting stuff with low binding energy into stuff with high binding energy, releasing heat.

note: the term "binding energy" has always screwed me up. The term evokes in my brain the image of energy that is there which is waiting to be released: but that's precisely backwards.

Could this outpace entropy/heat death?

That is heat death. Heat death means that there's nothing left to create heat with. When all the stuff with low binding energy is converted into stuff with high binding energy, and there is no stuff with low binding energy left, that's heat death: because you can't create more heat.

Note that the universe is not guaranteed to reach heat death. For instance, the expansion of the universe has a cooling effect on all the stuff in it. The cosmological red shift means that as space expands, light loses energy. It might be emitted as blue light, which has high energy, but might reach its destination as red light, which has low energy. Eventually, this will cool all matter in the universe to absolute zero, effectively ending all active processes. This is called the Big Freeze. The terminology is confusing, but it's important to remember that the Big Freeze is entirely different than Heat Death.

It boils down to stability in all cases. There are various interacting forces at the nuclear scale, but all reactions occur because of local equilibrium between these forces. Off the top of my head, I can only think of the proton to neutron ratio (i.e. weak nuclear vs. electromagnetism), but there are more intricate details as to why certain elements/isotopes are more or less stable.

edit: Here is a relevant source on both proton:neutron stability, and also a little tidbit about how protons/neutrons also fill nuclear shells similar to (but not the same as) electron shells, resulting in "magic numbers" of protons/neutrons which happen to be way more stable.

It's called the Stelliferous Era, and is 1015 years long, after that all the stars will be out of fuel, and dead. Next comes the Degenerate Era which is quite dreary but with occasional flashes of light as the occasional black hole eats a dead star and creates a temporary accretion disk. Meanwhile proton decay dismantles all the matter in the universe, including planets, stars, galaxies ect... Until everything made of matter is gone at about 1040 years. After that its the Black Hole Era, the era in which only black holes exist. And they exist only because they aren't made of matter, but they are decaying through Hawking radiation. So even they will disappear too, and after 1092 years the last object of any kind will cease to exist, leaving the universe as a thin soup of electrons, positions and neutrinos. And that's it, the end. What's 1092 years though? It seems like a long time, but compared to 101000 or 1010000000000000000000 years its nothing. So enjoy it while you can, diamonds are not forever, and neither is anything else.